Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long lifespan and low self-discharge are commercially available and widely used.
In addition, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles (EVs) and hybrid electric vehicles (HEVs) as substitutes for vehicles using fossil fuels such as gasoline vehicles and diesel vehicles which are main factors of air pollution. These electric vehicles generally use nickel hydride metal (Ni-MH) secondary batteries as power sources of with electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. However, a great deal of study associated with use of lithium secondary batteries with high energy density and discharge voltage is currently underway and some of them are commercially available.
In particular, lithium secondary batteries used for electric vehicles should have high energy density, exhibit great power within a short time and be used under harsh conditions for 10 years or longer, thus requiring considerably superior stability and long lifespan, as compared to conventional small lithium secondary batteries.
Conventional lithium secondary batteries generally utilize a lithium cobalt composite oxide having a layered structure for a cathode and a graphite-based material for an anode. However, such lithium cobalt composite oxide is disadvantageously unsuitable for electric vehicles in terms of presence of extremely expensive cobalt as a main element and low safety. Accordingly, lithium manganese composite oxide having a spinel structure containing manganese that is cheap and has superior safety is suitable for use as a cathode of lithium ion secondary batteries for electric vehicles.
However, lithium manganese composite oxides cause deterioration in battery properties since manganese is released into an electrolyte due to affection of the electrolyte when stored at high temperature. Accordingly, there is a need for a solution to this phenomenon. In addition, as compared to conventional lithium cobalt composite oxide or lithium nickel composite oxide, lithium manganese composite oxides have a disadvantage of low capacity per unit weight, thus having a limitation of an increase in capacity per battery weight. Lithium manganese composite oxide should be used in combination with battery design capable of solving this phenomenon in order to allow the same to be practically available as a power source of electric vehicles.
In order to solve these disadvantages, layered mixed metal oxides, LiNixMnyCozO2 (x+y+z=1) and the like are used, but they cannot secure satisfactory stability yet. Surface-treatment is attempted in order to solve this disadvantage, but problems such as increase in price which is one of the most important problems in the battery market such as electric vehicles occur due to the necessity of additional processes.